1. Field of the Invention
The invention relates generally to the field of communication networks. More particularly, the invention relates to the transport of digital data over subscriber loops and ATM networks. Specifically, the invention relates to deploying G.shdsl in backhaul applications.
2. Discussion of the Related Art
G.shdsl represents an international standard for transporting digital data over subscriber loops. The G.shdsl standard is established by the ITU-T as G.991.2. It provides a method for transporting a full-duplex bit-stream of up to 2.3 Mbps over short loops. The standard provides for operation that is rate-adaptive in nature, supporting payload rates ranging from 2.3 Mbps over 6 kft (26-AWG) loops to 192 kbps over loops as long as 18 kft (again, 26-AWG). A T1 rate (1.544 Mbps) can be supported over loops as long as 9 kft (26-AWG). This data distance profile (DDP) takes into consideration very high levels of cross-talk from adjacent loops in the same binder group. In conditions of low-noise, the capacity is much greater.
The encoding method specified for G.shdsl, known as trellis-coded pulse amplitude modulation (“TC-PAM”), is well suited for combating interference while being spectrally friendly with respect to other services carried over loops in the same binder group. The reach at a given data rate is longer when there is less interference. Several chip manufacturers provide proprietary extensions of the G.shdsl standard for improving the DDP.
There are numerous situations where a digital bit-stream needs to be delivered between a location, such as a cellular base-station or remote access multiplexer, and a central office. The bit-stream is usually a DS1 (“T1”; 1.544 Mbps) or DS3 (“T3”; 44.736 Mbps). More often than not, the DS1 is a framed signal, with a payload of 1.536 Mbps organized as a collection of DS0s; the DS3 is usually formatted with ATM cells and is “partially” full, corresponding to a data rate utilization of about 10 to 15 Mbps (filler ATM cells and DS3-specific overhead bring the rate up to 44.736 Mbps).
Referring to FIG. 1, a bock diagram of a DS3 backhaul application scenario is depicted. FIG. 1 depicts a situation where a competitive local exchange carrier (CLEC) has deployed a remote digital subscriber line access multiplexer (DSLAM) 110 for serving subscribers at a distance from the central office (CO). The CLEC would most likely deploy other equipment, such as DSLAMs and aggregation devices in the CO, renting space from the incumbent local exchange carrier (ILEC) in what is called a collocation arrangement (COLO). A plurality of subscriber DSL lines 100 is coupled to a remote DSLAM 110. The remote DSLAM 110 is coupled to a DS3 line 130 in a DS3 facility 131 via a DS3 connection 120. The DS3 line 130 is one of a plurality of lines 140. The plurality of lines 140 is coupled to an aggregation device 150.
The network side interface of the remote DSLAM 110 is a DS3 formatted stream of ATM cells, and, depending on the services provided and number of subscribers served, is most likely to be only partially occupied (“fractional-DS3”). However, the CLEC 160 must lease a complete DS3 line 130 from the ILEC to interconnect the remote DSLAM 110 to COLO-based aggregation equipment. The cost to the CLEC 160 for the leased DS3 line 130 can be quite high, depending on distance and market (geographic location). The business constraint of having to lease a complete DS3 line for a fractional application represents a problem (inefficiency) for the CLEC. What is needed is an approach that is less costly for the CLEC.
Another problem with this technology has been that, generally speaking, the capacity of a loop measured in terms of bit-rate, decreases as the loop length increases. It is not uncommon for the required capacity (demand) to exceed the carrying capacity. For example, in the case of DS1 backhaul, the objective is to transport 1.544 Mbps (or 1.536 Mbps) over the loop. If the length of the loop is 15 kft (26 AWG), then such a loop is incapable of achieving this objective since the length limits the rate to 192 kbps discussed above.
One unsatisfactory approach in an attempt to solve the above-discussed capacity problem involves deploying repeaters. The G.shdsl standard explicitly describes repeater operation. However, repeaters can be expensive, and their installation is not always feasible depending on geography and power requirements. What is needed is a solution that meets the above-discussed requirements in a more cost-effective manner.
Heretofore, the requirements for deploying G.shdsl in a backhaul application over long loops in a cost-effective and practical manner have not been fully met. What is needed is a solution that addresses these requirements.